Method of processing selected surfaces in a semiconductor process chamber based on a temperature differential between surfaces

ABSTRACT

The present invention relates to a method of processing selected surfaces in a semiconductor process chamber by creating a temperature differential between the selected surfaces and contacting the surfaces with a reactant that preferentially react with a surface at one end of the temperature differential relative to the other selected surface(s). More particularly, the invention relates to the use of nitrogen trifluoride (NF 3 ) gas for in situ cleaning of cold wall process chambers such as Rapid thermal Chemical Vaporization (“RTCVD”) systems.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of processing aselected surface in a semiconductor process chamber by contacting theselected surface with a reactant that reacts preferentially with theselected surface relative to another surface when the selected surfaceis at a different temperature from the other surface. The invention isparticularly suited for use in rapid thermal processing (RTP) systems.More particularly, the invention relates to the use of nitrogentrifluoride (“NF₃”) gas to clean selected surfaces in RTP processchambers, including in Rapid Thermal Chemical Vapor Deposition (RTCVD)process chambers.

[0002] During the course of semiconductor device fabrication, a waferundergoes many steps of processing. Some steps involve heating the waferhundreds of degrees Celsius above ambient temperature. Unnecessaryheating may lead to defective devices being formed on the wafer. Onereason for this is that heat causes substances, such as implanted ions,to migrate outside their intended location on the wafer. Migration andother things affected by heating are compounded by the length of heatingtime and by the number of heat steps. To avoid the problems associatedwith wafer heating, it is advantageous to use processes that minimizethe time that a wafer is kept at an elevated temperature. This is theprinciple of conserving the thermal budget of the wafer.

[0003] In a conventional process chamber using a tube furnace, forexample, it might take several minutes to raise the temperature of thewafer to a desired level. Because the walls and other surfaces in suchprocess chambers are directly heated along with the wafer, such systemscan be referred to as “hot wall” systems. The heated walls and othersurfaces in the chamber also increase the wafer's cooling time. Thus, inhot wall process chambers a wafer may be subject to prolonged heatingand cooling.

[0004] In contrast, RTP process chambers use a radiant energy source,such as high intensity lamps, to rapidly heat a wafer to a desiredtemperature in a matter of seconds. The energy source may raise thewafer's surface temperature by 75-125° C. per second. Other surfaces inthe chamber are not generally heated by the radiant energy—for examplequartz liners and windows in the chamber may be transmissive of theradiant energy frequencies or the energy may not be directed onto thesurfaces. Because the radiant energy source does not directly heat thesesurfaces, RTP systems may be referred to as “cold wall” systems.

[0005] Because the rapid heating and cooling in RTP systems conserve awafer's thermal budget, it was expected that RTP systems would be widelyadopted for every heat-based process. However, RTP systems have certaindisadvantages that have not been adequately addressed. One significantdisadvantage is that radiant energy may not be absorbed uniformly by awafer. This results in temperature variations in and across the wafer.Temperature variations lead to non-uniform process results. RTP systemstherefore require sensitive systems for monitoring and controlling thetemperature of the wafer. One useful system uses a parallel plate with areflective coating that reflects energy to the backside of the wafer. Anoptic fiber collects and transmits the wafer's backside emissivity to adetector that translates the frequencies of energy to a temperaturereading. Unfortunately, a significant disadvantage of this temperaturemonitoring system is its vulnerability to certain cleaning processescarried out in an RTP process chamber.

[0006] This problem and certain others addressed by the presentinvention are illustrated in the following example: the deposition ofpolysilicon on a wafer in an RTCVD process chamber. Deposition ofpolysilicon is a known step in the fabrication of certain semiconductordevices. One notable process using polysilicon is the formation ofhemispherical grained silicon (“HSG”). HSG formations enhance thestorage capacitance in storage devices such as Dynamic Random AccessMemory Arrays (“DRAMS”). Poor, irregular HSG formations result inunclean RTCVD process chambers. (Methods for forming HSG are described,for example, in U.S. Pat. Nos. 5,634,974 and 5,759,262 which are herebyincorporated by reference as if set forth in their entirety.).

[0007] An undesired side effect from polysilicon deposition is that thepolysilicon is deposited not only on the wafer but also on othersurfaces in the process chamber. For example, in RTCVD process chambers,it accumulates on, among other things, the top quartz window and quartzliner of the chamber. A build-up of polysilicon impedes the transmissionof radiant energy through the window onto the wafer. It may also lead tohigh particle counts in the chamber, lowering production yields.Therefore, periodic cleaning of the process chamber is necessary.

[0008] NF₃ is a powerful etchant, effectively etching polysilicon andcertain other substances used in semiconductor processing. Use of NF₃for in-situ cleaning of a non-RTP semiconductor process chambers isdescribed in U.S. Pat. No. 5,797,195, to Hulling et al, entitled“Nitrogen Trifluoride Thermal Cleaning Apparatus and Process.” The '195patent describes a “hot wall” system for cleaning semiconductorfabrication equipment, including quartzware parts. NF₃ gas is heatedfrom approximately 100° C. to 650° C. by the existing heat source forthe process chamber. At the same time, the heat source also heats theother surfaces in the process chamber, which surfaces the NF₃ gas isfree to contact. The '195 patent does not teach or suggest that thedescribed cleaning system is suitable for use in “cold wall”, RTPsystems, including RTCVD systems.

[0009] It would be desirable to use NF₃ gas to clean RTP processchambers, particularly those used to deposit polysilicon in HSGproduction. However, NF₃'s high reactivity has so far limited itsusefulness in this regard. NF₃ cleaning, until the present invention,has not proved suitable for use in cold wall systems, such as RTCVDsystems, because certain components in RTCVD chambers may be damaged bythe high activity of NF₃ at higher temperatures.

[0010] As the temperature of NF₃ increases, its reactivity increases,converting to reactive species in the form of ionic fluorine and/or freefluorine. The temperatures that occur in RTCVD processing can exceed750° C. At such temperatures, NF₃ is so corrosive that it attacksstainless steel, quartz, and silicon surfaces, damaging criticalcomponents found in RTCVD process chambers. As mentioned,temperature-monitoring systems based on wafer emissivity areparticularly vulnerable to damage. NF₃'s action on such unintendedtargets may also create unacceptably high particle counts in the processchamber, lowering production yields.

[0011] If the NF₃ cleaning is performed at lower temperatures, theetching rate is too low to effectively clean the system. This is becausemost of the hardware inside the process chamber, including the topquartz window, is heated by heat transmitted from the wafer, not bylight used to heat the wafer. Therefore, if the temperature of the waferis low, the top quartz window is even cooler. This results inunacceptably low etching rates.

[0012] For the foregoing reasons, a method is needed that allows areactant introduced into an RTP process chamber to act preferentially oncertain surfaces. Among other things, such an improved method wouldallow strong etchants, such as NF₃, to clean polysilicon deposits fromprocess chambers without damaging sensitive components in the chamber.

SUMMARY OF THE INVENTION

[0013] The present invention overcomes the disadvantages of the priorart by providing a method by which a reactant may favor a reaction withselected surfaces in a semiconductor process chamber. In so doing, thepresent invention overcomes the problem of using a reactant that wouldact desirably on some surfaces and detrimentally on other surfaces.

[0014] The present invention provides a method that will facilitate thecleaning of RTP process chambers, thereby better facilitating theadoption and use of RTP systems. More particularly, the presentinvention overcomes the inherent disadvantages of using a powerfuletchant, such as NF₃ gas to clean an RTP process chamber.

[0015] The present invention also overcomes the problems of highparticle counts and lowered production yields that result if NF₃ gas orother strong etchant is used to clean RTP process chambers. Accordingly,the present invention also improves the process of HSG formation in RTPprocess chambers.

[0016] In overcoming the aforementioned disadvantages in the art, thepresent invention improves the efficiency and yield of semiconductordevice production. Particularly, it improves the efficiency and yield ofproducing devices in RTP process chambers. More particularly, itimproves the efficiency and yield of producing HSG formations on wafersusing RTCVD systems.

[0017] One embodiment of the invention is a method of processingsurfaces in a semiconductor process chamber, comprising: selecting twodifferent surfaces in the process chamber, each surface at a giventemperature being capable of reacting with a reactant introduced intothe chamber; creating a predetermined temperature differential betweenthe selected surfaces by allowing a heated object in the chamber totransfer heat to one selected surface so that that surface becomes thesurface at the higher end of the temperature differential; contactingthe selected surfaces with a reactant present in the chamber during thepredetermined temperature differential between the selected surfaces;and allowing sufficient time for the reactant to react preferentiallywith one surface to a predetermined degree. The reactant may be removedfrom the chamber after the reactant has reacted with a surface to apredetermined degree.

[0018] A method according to the present invention may also includerepeating the foregoing steps-following the processing of apredetermined number of work objects in the chamber. The foregoingmethod also provides for the removal of deposits composed substantiallyof polysilicon from the surface at the higher end of the temperaturedifferential.

[0019] In the embodiments of the present invention, a selected surfaceat the lower end of the temperature differential may be cooled by acooling means. The cooling means may be a fluid cooling system inconductive communication with the surface at the lower end of thetemperature differential. Components of a temperature measurement systemin the process chamber may be cooled to protect them against action byan etchant or other reactant.

[0020] The present invention may provide for the process chamber toinclude a reflectivity plate comprising a selected surface. Thereflectivity plate is kept at the lower end of the temperaturedifferential during cleaning by an NF₃ gas or other etchant. By keepingthe reflectivity plate at a lower temperature, it is protected frombeing damaged by an etchant that prefers reacting with other surfaces inthe chamber kept at a higher temperature.

[0021] The present invention may also provide that a radiant energysource transmits energy to a work object thereby heating it. The surfaceof the heated work object radiates heat to a selected surface thatbecomes heated to the higher end of a temperature differential. Theinvention also provides that polysilicon may be etched off a surfacethat is transmissive of the radiant energy used to heat a work object inan RTP process chamber. One such transmissive surface may be the windowbetween the radiant energy source and a work object. In the presentinvention, the etching of the transmissive surfaces occurs after theyare heated by another object in the process chamber. That other objectmay be heated by the radiant energy source. Quartz surfaces in thechamber, including quartz liners and windows may be etched accordingly.

[0022] In another embodiment of the present invention, a method ofcleaning an RTP process chamber is provided that includes the steps ofheating a selected absorbent surface in the process chamber with energyfrom a radiant energy source, the radiant energy passing through atransmissive surface between the radiant energy source and the selectedsurface; allowing a selected transmissive surface in the chamber to heatby energy transferred from the selected absorbent surface, after theabsorbent surface is heated by the radiant energy source; and contactingthe heated transmissive surface with an etchant while there is apredetermined temperature differential between the selected transmissivesurface and another selected surface in the chamber; and allowingsufficient time for the etchant to react preferentially at thetransmissive surface to a predetermined degree relative to the otherselected surface. Preferably, the selected absorbent surface comprises awafer. The etchant in this embodiment may be NF₃ gas. The other surfacemay be on a component of a temperature measurement system, such as areflectivity plate. This embodiment is suitable for etching deposits onthe transmissive surface, including polysilicon deposits. Thisembodiment also provides that the other surface may be cooled by acooling means so that it is at the lower end of the temperaturedifferential. For certain processes, particularly cleaning polysilicondeposits with NF₃ gas or a similar etchant, it is advantageous to heatthe selected absorbent surface to at least about 650° C. to about 750°C. so that it transfers sufficient heat to the selected transmissivesurfaces to establish an appropriate temperature differential. Moreparticularly, NF₃ gas may be used to clean polysilicon from thetransmissive surface by providing a selected temperature differentialbetween the surfaces of at least about 200° C. to about 500° C., withthe temperature of the transmissive surface with the polysilicondeposits at the upper end of the differential and being at least about650° C.

[0023] In still another embodiment of the present invention, a method ofin situ cleaning of a process chamber is directed to running productionwafers through a process chamber for depositing silicon on the wafers;stopping production runs for cleaning the chamber when silicon hasdeposited on the liner or a window of the chamber to a predetermineddegree; heating a selected absorbent surface in the process chamber withenergy from a radiant energy source, the radiant energy passing througha transmissive surface between the radiant energy source and theselected surface; allowing a transmissive surface in the chamber to heatby energy transferred from the selected absorbent surface; and, after aselected temperature differential has been established between thetransmissive surface and another surface in the chamber, contacting anetchant present in the chamber with deposits on the transmissivesurface. In this embodiment, as well as other embodiments, the selectedabsorbent surface may be on a silicon-based wafer. And the etchant maybe NF₃ gas. In this embodiment, as well as others, the selectedtemperature differential between the selected surface and other surfacemay be at least about 200° C. Preferably, the selected temperaturedifferential between the surfaces is from about 200° C. to 500° C. Thetemperature of the surface with the polysilicon or other material to beetched is at the upper end of the differential. A useful upper-endtemperature for etching the selected surface with NF₃ gas is at leastabout 600° C. A preferable upper-end temperature for etching with NF₃gas is about 600° C. to about 750° C. However, in other embodiments, thetemperature of the surface at the upper end of the temperaturedifferential may be in excess of about 1000° C., depending on thematerials to be reacted.

[0024] Another embodiment of the present invention contemplates cleaninga semiconductor process chamber using a gas etchant. This embodiment isdirected to heating an absorbent surface in the process chamber heat byenergy from a radiant energy source, the absorbent surface being heatedfrom about 400° C. to about 1500° C.; allowing a selected surface in thechamber to heat by energy transferred from the heated absorbent surface;and cooling another surface in the chamber so that there is atemperature differential between the cooled surface and the heatedselected surface such that an etchant in the chamber reactspreferentially with the heated. Here again, the etchant may comprise NF₃gas. The temperature differential between the surfaces is alsopreferably at least about 200° C.

[0025] As in other embodiments, the etchant may be NF₃ gas. Again, asuitable temperature differential between the selected surface and othersurface is at least about 200° C.

[0026] The embodiments of the present invention are suitable for use ina cold-wall process chamber, particularly an RTCVD process chamber, usedto form HSG capacitors on production wafers. When cleaning a processchamber of polysilicon deposits the polysilicon/silicon dioxide etchingselectivity ratio may be in the range of about 4/1 to about 7/1, whichwill allow cleaning without unacceptable etching of other components inthe chamber.

[0027] Other embodiments will be apparent to persons skilled in the art,which embodiments do not depart from the spirit and scope of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 shows an overview of an RTCVD semiconductor processingsystem in which the method of the present invention may be practiced.

[0029]FIG. 2 is a top view of certain features of the processing chamberof the processing system of FIG. 1.

[0030]FIG. 3 shows process control units for controlling certain stepsand conditions of the methods of the present invention.

[0031]FIG. 4 is a plot of the temperature measured at the wafer/platformin an RTCVD chamber as a function of time that helps illustrate certainprinciples of an embodiment of the present invention.

[0032]FIG. 5 is plot of the ratios of the NF₃ etch rates of polysiliconand silicon dioxide as a function of temperature to help illustratecertain principles of an embodiment of the present invention.

[0033]FIG. 6 is a plot of the NF₃ etch rate of polysilicon to helpillustrate certain principles of an embodiment of the present invention.

[0034]FIG. 7 is a plot of the NF₃ etch rate of silicon dioxide thathelps illustrate certain principles of an embodiment of the presentinvention.

[0035]FIG. 8 is a plot of the average number of particles per waferafter a process chamber was cleaned with NF₃ gas according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0036] In this invention, a process chamber is a vessel where processesare being performed to fabricate discrete devices or circuits on a workobject. The process chamber may be used to oxidize, etch, dope, deposit,implant or pattern materials in or on the work object, as well as toclean, prepare and condition the work object, among other things. Thepresent invention is preferably used to process work objects in an RTPprocess chamber for chemical deposition, where the work object is heatedto a higher temperature than surrounding walls in the chamber.

[0037] “Work object” means objects, including, wafers (includingproduction, dummy, or pmon), die and packaged parts, incorporating, inwhole or part, silicon substrates, and other known or discoveredsemiconductor materials, components, and assemblies, including, forexample, silicon-on-insulator (SOI), silicon-on-sapphire (SOS), thinfilm transistor (TFT) materials, or germanium, periodic group III-IVmaterials, II-VI materials, hetero-materials (II, III, V, VI), andconductive glasses.

[0038] The following description of a process chamber for Rapid ThermalChemical Vapor Deposition (“RTCVD”) processing of a semiconductor workobject is intended to identify basic hardware features relevant to thediscussion of the principles of the present invention. However, it willbe apparent to those skilled in the art that the principles of thepresent invention may be applied to other kinds of semiconductor processchambers, particularly other RTP chambers for Plasma Enhanced ChemicalVapor Deposition (“PEVCD”), Low Pressure Chemical Vapor Deposition(“LPCVD”), and other process systems. Likewise, while the principles ofthis invention may be discussed in terms of depositing polysilicon on,and etching it from, silicon based wafers or other surfaces in an RTCVDchamber, or in terms of using NF₃ gas as a reactant, the invention'sprinciples, and the claims appended hereto, may apply to otherprocesses, processing chambers, and reactants for semiconductor workobjects.

[0039] Now using the example of an RTCVD system, such chambers areadapted for cycled rapid heating and cooling of sequentially introducedwork objects. In an RTCVD chamber, a radiant energy source emits burstsof energy directed to a work object, rapidly heating the object to asubstantially higher temperature than some or all of the surroundingsurfaces in the chamber. A chemical reagent, usually a gas, is thenintroduced into the chamber to effect some process on the heated workobject. An example process discussed herein is the deposition ofpolysilicon on a wafer to create HSG formations.

[0040] FIGS. 1-3 show basic details of an exemplary RTCVD chamber 10.The chamber 10 includes a cylindrical container 12 made of stainlesssteel, for example. A cylindrical liner 14 is disposed within container12. The liner 14 is typically made of quartz. The container 12 has anopening at its top end for receiving a window 16. In RTCVD chambers,window 16 is typically made of quartz. A “shower head” 18, with aplurality of apertures for focusing radiant energy on an object in thechamber may optionally be disposed in an opening at the top end of liner14. Showerhead 18 is also typically made of quartz.

[0041] A radiant energy source 19, including one or more high intensityelements 20 for producing electromagnetic radiation of a desiredfrequency and intensity sufficient to rapidly heat a silicon wafer orother semiconductor work object is positioned proximate to quartz window16. The window 16 is transmissive of radiant energy from radiant energysource 19. Elements 20 typically may be tungsten-halogen lamps or otherlamps capable of rapidly heating a semiconductor work object. Otherpossible radiant energy sources include plasma arc lamps, graphiteheaters, and microwave units.

[0042] Lamps 20, quartz window 16, and optional showerhead 18, arearranged so that radiant energy is transmitted through window 16 andoptional shower head 18 to a wafer or other work object 22 disposed on awafer platform 26 at the bottom end of liner 14.

[0043] The wafer platform 26 (also referred to as a “cold head”) forholding wafer 22 includes a reflectivity plate 24 disposed at the top ofplatform 26 directly below wafer 22. The reflectivity plate 24 typicallycomprises a parallel plate with a reflective coating that reflectsenergy received from radiant energy source 20 onto the backside of wafer22. The reflectivity plate 24 also directs energy emitted from thebackside of the wafer into one or more optical fibers 28. The wafer isheld by the wafer platform on a pedestal 25 slightly above reflectivityplate 24. A spacing of about 1 cm is suitable. Optical fiber 28transmits the collected energy to a detector 30. The temperature ofplatform 26, including the temperature of reflectivity plate 24, may becooled or heated by a temperature control means discussed below. Forexample, the temperature of the wafer platform 26 may be conductivelycontrolled by fluid flow through one or more channels 35 in platform 26.The rate of cooling or heating may be adjusted by changing the rate offlow or the temperature of the fluid.

[0044] One or more fluid lines 34 and 36 may be used to circulate afluid, such as water, into a channel 35 for cooling or heating areas ofthe process chamber in conductive communication with channel 35.

[0045] The temperature measurement system includes reflectivity plate24, detector 30 and one or more optical fibers 28 coupled to a detector30 for converting the optical signals into a temperature reading. Thedetector 30 is capable of translating the frequencies of theelectromagnetic radiation from wafer 22 into a signal or readingcorresponding to the temperature of the wafer. The signal may be furthertransmitted to a central processing unit (CPU) 42 or a display means,such as a gauge. The signal may be analog or digital.

[0046] One or more lines 32 may be in communication with the inside ofchamber 10 to introduce or remove gas or liquid reagents into thechamber for cleaning the inner surfaces of the chamber 10 or processingobject 22. One such line is in communication with a reactant source 38.One reactant contemplated by the present invention, discussed in moredetail below, is NF₃ gas.

[0047] Gas and fluid flow in the system may be controlled by appropriatepressure regulators, mass flow meters, valves, and timers (not shown).These control means may be in communication with CPU 42 or anotherprocessing unit.

[0048] One or more pumps 40 may be in communication with one or more gasor liquid reagent sources to introduce, remove, and/or circulate areagent through chamber 10. For example, pump 40 may be used to evacuatechamber 10 of gas, other reagents, or by-products present in chamber 10during or after cleaning.

[0049] The central processing unit (“CPU”) 42 may be used to controlvarious processes in chamber 10. The CPU 42 may be in the form of a PCcomputer, workstation, or other computer system known in the art. Forexample, based on signals received from detector 30, CPU communicateswith a radiant energy controller 44 for controlling radiant energysource 19. CPU 42 could instruct radiant energy source 19 to turn on oroff based on temperature measurement signals received from detector 30.

[0050] The CPU 42 may also be in communication with fluid linecontroller 46 to control the level and temperature of fluid flow intochamber 10. In addition to platform 26 and reflectivity plate 24 beingin conductive communication with the cooling means, chamber 10 could beprovided with additional fluid lines and channels of fluid flow toconductively control the temperature of other areas of the processchamber. CPU 42 or other CPUs may also communicate with other systemcomponents, including: etchant source controller 48 for controlling theflow of etchant gas into chamber 10; pump controller 50 for controllingthe pumping of materials into or out of chamber 10 or to increase orlower the pressure in the chamber; and other mechanisms controlling ormonitoring process steps or conditions. Of course, virtually any aspectof the process chamber and processes carried out therein, may bemonitored and controlled, in whole or part, manually, by CPU 42, or byother CPUs integrated into the system.

[0051] One possible embodiment of the present invention will now bedescribed. It is a method for cleaning deposits off surfaces in a RTCVDchamber used to process a semiconductor work object. The method isparticularly suitable for cleaning an RTCVD chamber. And the cleaningmethod is particularly well suited for cleaning an RTCVD process chamberused to deposit polysilicon on a wafer to form selective HSG formationsfor memory storage devices, such as a DRAM device.

[0052] In the method, radiant energy source 19 is activated for one ormore predetermined intervals to heat work object 22 to a desiredtemperature. (The use of radiant energy to heat a wafer object apredetermined degree is well known in the art and not discussed furtherherein.) The inner surfaces of the RTCVD chamber containing the waferare substantially non-absorbent and/or transmissive of the radiantenergy source used to heat the wafer. In a typical RTCVD chamber, thesesurfaces include the inner surfaces of liner 14, window 16 and, ifpresent, shower head 18, all of which are usually made of quartz.Therefore, there is comparatively little, if any, direct heating ofthese surfaces by radiant energy source 19.

[0053] After the wafer is heated to a desired degree, the radiant energysource is switched off or set to a lowered level. At this point there isa first temperature differential between the wafer and the othersurfaces in the chamber. The heated wafer then transfers its absorbedenergy to the other components and parts in the chamber, including thequartz window. Wafer platform 26, which includes reflectivity plate 24,is not substantially affected by this heat transfer because it is cooledin a typical RTCVD system, such as shown in FIGS. 1-3, for example.Consequently, as the wafer transfers its heat to the quartz liner,window, and other components, those components reach a highertemperature than platform 26. Thus, during the heat transfer, a secondtemperature differential occurs between the surfaces in the chamber thathave absorbed heat from the wafer and the surfaces in the cold head thatare fluid cooled.

[0054] A temperature dependent reactant may be introduced at either thefirst or the second temperature differential to preferentially react atone more of the surfaces at one end of a temperature differential. Ifthe reactant is introduced during the first temperature differential,the reactant will react preferentially with the wafer, if the reactant'sreactivity increases with temperature. This reaction during the firsttemperature differential is of course a known technique; it is theintended purpose of RTP systems. (As a general rule reaction ratesincrease with temperature, but there are situations where reaction ratesmay decrease with increases in temperature: for example, a reactant maystart to decompose beyond a particular temperature.).

[0055] Accordingly, in RTP systems, the present invention generallypertains to the second temperature differential based on heat transferfrom the wafer or another energy absorbent surface to which the radiantenergy source directs energy. As used herein, the term “temperaturedifferential” means this second temperature differential, unless thecontext indicates that the first temperature differential is meant.

[0056] After the temperature differential is created, a reactantintroduced into the chamber is contacted with different surfaces whilethere is an appropriate temperature differential between the surfaces.The reactant may be introduced once the temperature differential isestablished. Alternatively, the temperature differential may be createdin the presence of the reactant, if the reactant does not adverselyaffect a selected surface before or during the creation of thetemperature differential. There is a different rate of reaction at eachsurface during the temperature differential. The reaction is allowed tocontinue until the reactant has produced the desired process results onselected surfaces. The reactant may be removed from the chamber and/orthe temperature at the selected surfaces may be adjusted to stop or slowthe reaction at a selected surface. The reaction may also be carried outin one or more cycles.

[0057] NF₃ gas may be used as the reactant to clean an RTP processchamber. It reacts preferentially on surfaces at the higher end of thetemperature range in an RTCVD process chamber. The NF₃ gas may reactwith a number of substrates in a process chamber, including siliconbased substrates such as amorphous silicon, polysilicon, and siliconoxides. Other substances reactive with NF₃ are well known in the art,and include SiC, certain metals, etc. In addition to NF₃, other suitableetchants include HCl, Cl₂, other Cl containing gases, etc.

[0058] A dummy wafer may be used as the energy absorbent surface in anNF₃ etch process if production wafers being run through the chamberwould have a low tolerance to any etching or otherwise would be damagedby the cleaning process. Likewise, the energy absorbent surface need noteven be on the work object. It can be any object in the chamber that isdesignated to receive energy from the radiant energy source and whichcan transfer the energy to a selected surface to be processed.

[0059] There should be an appropriate spread between the higher andlower ends of the temperature differential. The appropriate spread willvary from situation to situation. Some factors that need to beconsidered in determining an appropriate spread include: the reactionrate of the reactants on the particular surfaces at temperatures andpressures maintained or maintainable in the process chamber, and theacceptable degree to which one or both surfaces will react. Using knowntechniques, persons of skill in the art generally may determine thereaction rates for a reactant at given temperatures on the substrate, orat given temperatures for different substrates. Using such information,a temperature differential of an appropriate spread may be determinedand maintained for selected surfaces in the process chamber. For examplethe relative NF₃ etch rates for silicon and silicon dioxide are given inthe example below. These etch rates show that NF₃ will preferentiallyetch polysilicon deposits on the quartz liner of an RTCVD chamberwithout unacceptable etching of any exposed quartz surfaces. In general,a temperature differential of at least 200° C. will be appropriate forNF₃ gas etching. Preferably the temperature differential is about 200°C. to about 500° C. when using NF₃ gas to clean quartz of polysilicon orsimilar deposits. The surface with the deposits, at the upper end of thetemperature differential, should be at least about 600° C. A favorabletemperature range has been found to be about 600° C. to about 750° C.

[0060] To provide such temperatures, a wafer or other object with anabsorbent surface may need to be heated to a higher temperature than thedesired upper end of the temperature differential in order to compensatefor inefficiencies in the heat transfer. The absorbent surface can beheated by the radiant source up to at least about 1500° C. It isbelieved that if the absorbent surface is heated between about 400° C.to about 1500° C., there should be suitable heat transfer to cover mostapplications where the upper end of the desired temperature differentialfalls within this range.

[0061] In addition to the RTP heating systems, temperature differentialsmay also be created between selected surfaces in process chambers byother heating or cooling means, alone or in combination, including:conventional heat transfer means disposed in the process chamber,including conduction or convection heating systems; induction RF,plasma, and other radiant energy sources, including ultraviolet;conduction or convection cooling systems, including, refrigerationsystems, fans, cooling fins, heat sinks; and other known forms of director indirect heat transfer to selected surfaces that may be implementedin a process chamber.

EXAMPLE EMBODIMENT OF THE INVENTION Cycled in Situ Clean of an RTCVDProcess Chamber Using NF₃ Gas

[0062] The following is one example embodiment of the invention, whichwas tested by the inventor. The example is intended to help illustrateand further describe the foregoing principles of the invention. Theinformation in the example embodiment is believed to be reasonablycomplete and accurate. Notwithstanding the inclusion of this example,the inventor believes the patentability of the present invention hasbeen established independently of this example by the foregoingdescription and attached figures.

[0063] The following testing was carried out to determine if NF₃ gascould be used to clean an AGI Integra Pro clustered RTCVD processchamber of polysilicon deposits. The Integra Pro is manufactured by AGAssociate Israel. The test evaluated whether the cleaning process wouldcause damage to the temperature measurement system or other componentsof the process chamber. The test also evaluated the impact of thecleaning process on production processes and yields, including whetherunacceptable particle counts or other production problems resulted.

[0064] The Integra Pro process chamber was set up according tospecifications of the assignee of this invention. FIGS. 1-3 generallyillustrate relevant features of the Integra Pro. As seen in the Figures,a reflectivity plate collects the backside emissivity of a waferdisposed about 1 cm above the reflectivity plate. The reflectivity plateis disposed in a cold head with water-cooled channels that conductivelycool the reflectivity plate. The coating of the reflectivity plate isdegraded by NF₃ at temperatures in excess of about 600° C.

[0065] In this example, time and temperature measurements are based onthe temperature of a dummy wafer. In the tests described in thisexample, it was assumed that the temperature of the reflectivity platewas substantially the same as the wafer, although the plate'stemperature is actually somewhat lower because the wafer is not indirect contact with the cold head, while the reflectivity plate is. Anytemperature difference, however, is not believed to be substantial interms of the results and conclusions from this example.

[0066] The Integra Pro RTCVD process chamber was used to depositpolysilicon on eight-inch silicon wafers to form selective HemisphericalGrained Silicon (“HSG”) formations for 16K or 64K DRAMS. Wafers wereprocessed individually in the chamber. At intervals of about every 300production wafers, it was desirable to clean polysilicon thataccumulated on permanent or temporary surfaces in the process chamber.The quartz window and liner of the chamber are included among suchsurfaces.

[0067] Accordingly, after about 300 wafers were run, a dummy wafer wasintroduced into the process chamber. The dummy wafer was the same as aproduction wafer, except it was unpatterned. At the start of thecleaning cycle, the base temperature in the chamber was about 450° C.Over about five minutes, the temperature was ramped up to about 675° C.under a vacuum, creating a pressure of about 10⁻⁷ Torr.

[0068]FIG. 4 is a plot of the temperature of the wafer over time duringthe cleaning cycle. The introduction of the wafer into the chamberoccurs at zero on the time axis of FIG. 4. The temperature ramp-up undervacuum is shown from about 50 to about 100 seconds. Alternatively, thetemperature ramp-up could have been conducted under Nitrogen gas orother inert gas or gases. It also could have been conducted with NF₃present because overetching of dummy wafers by NF₃ poses little concern.

[0069] After the temperature ramp-up, there is a high temperature stepfor the wafer. This is shown in FIG. 4 from about 100 to about 300seconds. During the high temperature step, the top quartz window washeated by the energy radiating off the wafer. After about 300 seconds,the wafer platform (cold head) was cooled to about 500° C. NF₃ gas wasthen introduced into the chamber.

[0070] The gas was introduced into the chamber by a plurality of jetscircumferentially disposed in the top of the chamber, as shown in FIG.3.

[0071] The flow rate of NF₃ into the chamber was from about 1000-2000standard cubic centimeters per minute (“SCCM”), and the chamber pressurewas maintained at about 10⁻³ to about 10⁻⁴ Torr while the NF₃ gas wasflowing in the chamber. During NF₃ flow, a pump withdrew gas andresiduals through a port disposed near the bottom of the chamber. Theabove-stated pressure in the chamber was maintained by appropriatebalancing of the gas inflow rate against the gas outflow rate from pumppull. The gas flow corresponded to the period of about 300 to about 500seconds, seen in FIG. 4 as the lower temperature step of about 500° C.

[0072] During this lower temperature step, the temperature of the topquartz window was much higher than the cooled components of the chamber,particularly the reflectivity plate. (Temperature of quartz wasapproximated and not measured directly.) This was because thetemperature measurement system was water-cooled, but the top quartzwindow was not. Consequently, the NF₃ etched polysilicon on the quartzwindow with comparatively little or no etching of the temperaturemeasurement system. Etching of quartz begins at about 600-650° C. at thepressures stated herein. Thus, during the lower temperature step of thewafer, a temperature differential existed between the reflectivity plateand the quartz components in the chamber sufficient for the NF₃ to reactpreferentially at the quartz surface relative to the surface of thereflectivity plate. After about 500 seconds, the quartz cooled,diminishing the spread of the temperature differential, and ultimatelyreducing the etch rate to less than an effective rate. Therefore, NF₃flow was stopped at about 550 seconds on the time axis of FIG. 4.

[0073] A second cycle of the foregoing steps was performed. The secondcycle started with reheating the dummy wafer for a second hightemperature step of about 675° C. (about 550 to about 800 seconds onFIG. 4's time axis). The wafer platform was then cooled again. And NF₃gas flowed during a second lower temperature step of about 500° C.(about 800 to about 1000 seconds on FIG. 4's time axis). Additionalcycles could also have been performed as needed to further clean theprocess chamber.

[0074] Following the two cycles of cleaning with the dummy wafer, 25additional dummy wafers were sequentially run through the processchamber under production conditions for forming selective HSG. Thepurpose of running these dummy wafers was to recondition the chamber forproduction. During reconditioning, residual matter from the NF₃ cleaningwas removed from the chamber. The chamber was then visually inspectedand found to be free of polysilicon on the top quartz window and quartzliner. The chamber was then deemed ready for continued production.Production runs after the cleaning showed that particle levels did notexceed maximum acceptable levels. It also showed that selective HSGformation was good. These results are discussed in more detail below.

[0075]FIG. 5 shows the NF₃ etching rate ratio of polysilicon andoxidized silicon. The rates are based on measurement of SiO₂ on an 8″silicon wafer. These data indicate that at about 500° C., the lowertemperature step for the wafer in FIG. 4 (or conversely the highertemperature step for the quartz), the NF₃ polysilicon/silicon dioxideetching selectivity ratio is about 4/1 to about 7/1. The upper plot inthe graph with open data points shows the etch rate at about the edge ofthe wafer, the lower plot with darkened data points shows the etch rateat about the center of the wafer.

[0076]FIG. 6 shows a semi-log graph of the etch rate of polysilicon inangstroms per minute as a function of temperature (1000/°K). Open datapoints were from measurements taken at about the edge of an 8″ wafer;darkened points were measurements taken at about the center of thewafer. The flow rate of NF₃ into the AGI Integra Pro was 1000 SCCM.

[0077]FIG. 7 shows a semi-log graph of the etch rate of an oxide layeron the wafer in angstroms per minute as a function of temperature(1000/°K). Open data points were from measurements taken near the edgeof an 8″ wafer; darkened data points were measurements taken near thecenter of the wafer. The flow rate of NF₃ into the AGI Integra Pro was1000 SCCM.

[0078] A scanning electron microscope (SEM) was used to examine HSGformations on wafers run through the Integra Pro after NF₃ cleaningaccording to the foregoing example. The SEM images showed that both theHSG conversion and selectivity are good, producing uniform formations.

[0079]FIG. 8 shows average particle counts on pmon wafers introducedinto the chamber of an AGI Integra Pro after HSG processing of 1800 testwafers, with the NF₃ clean performed at an interval of every 300 testwafers. The wafers were as described in the above example. “Slot #” onthe x-axis refers to the slot on a cassette from which a wafer wasinserted into the chamber. As shown in FIG. 8, the average number ofparticles does not exceed acceptable levels after the NF₃ clean. (Countsunder 100 particles per pmon wafer were considered acceptable.) Thenumber of acceptable die per wafer also exceeded minimum standards andwas maintained at a consistent level.

[0080] A marathon run of about 3000 wafers using the foregoing NF₃cleaning procedure indicated that selective HSG capacitor formationsmaintain a 2× increase of capacitance over conventional smoothpolysilicon capacitor formations.

[0081] Persons skilled in the art will recognize the foregoingdescription and embodiments are not limitations but examples. It will berecognized by persons skilled in the art that many modifications andvariations to the present invention are possible that are still withinthe spirit and scope of the teachings and claims contained herein.

What is claimed:
 1. A method of processing surfaces in a semiconductorprocess chamber, comprising: selecting two different surfaces in theprocess chamber, each surface at a given temperature being capable ofreacting with a reactant introduced into the chamber; creating apredetermined temperature differential between the selected surfaces byallowing a heated object in the chamber to transfer heat to one selectedsurface so that that surface becomes the surface at the higher end ofthe temperature differential; contacting the selected surfaces with areactant during the predetermined temperature differential between theselected surfaces; and allowing sufficient time for the reactant toreact preferentially at one selected surface to a predetermined degreerelative to the other selected surface.
 2. The method of claim 1 furthercomprising the step of removing the reactant from the chamber after thereactant has reacted at the preferred surface to a predetermined degree.3. The method of claim 1 wherein the method steps are repeated in thechamber following the processing of a predetermined number of workobjects in the chamber.
 4. The method of claim 1 wherein the objectcomprises a work.
 5. The method of claim 1 wherein the surface at thelower end of the temperature differential is subject to cooling by acooling means.
 6. The method of claim 1 wherein the chamber includes areflectivity plate having the surface at the lower end of thetemperature differential, and the reactant does not react preferentiallyreact with the surface of the reflectivity plate.
 7. The method of claim4 wherein the selected surface at the higher end of the temperaturedifferential is a window surface through which radiant energy istransmitted to heat the work object, and the reactant reactspreferentially at the surface of the window.
 8. The method of claim 6wherein the reflectivity plate is subject to cooling to place thesurface of the reflectivity plate at the lower end of the temperaturedifferential.
 9. The method of claim 7 wherein the reactant comprisesNF₃ gas.
 10. The method of claim 1 wherein a radiant energy sourcetransmits energy to the object thereby heating it, the surface of theobject radiating the absorbed energy to a selected surface that becomesthe surface at the higher end of the temperature differential, thatselected surface being on a liner or window in the process chamber. 11.The method of claim 10 wherein the surface at the lower end of thetemperature differential is subject to cooling by a cooling means. 12.The method of claim 10 wherein the reactant comprises an etchant gas.13. The method of claim 12 wherein the etchant gas comprises NF₃. 14.The method of claim 10 wherein polysilicon deposits are being etchedpreferentially from the quartz liner or window surface at the higher endof the temperature differential.
 15. The method of claim 11 wherein thecooling means comprises a fluid cooling system in conductivecommunication with the surface at the lower end of the temperaturedifferential.
 16. The method of claim 11 wherein the cooling means is inconductive communication with at least one component of a temperaturemeasurement system.
 17. The method of claim 12 wherein the gas is beingused to clean polysilicon deposits off a quartz surface in the processchamber.
 18. The method of claim 12 wherein the surface heated to ahigher temperature by heat radiating from the work object istransmissive to the radiant energy used to heat the work object.
 19. Themethod of claim 18 wherein the transmissive surface comprises quartz.20. The method of claim 19 wherein the quartz surface comprises a quartzwindow positioned between a radiant energy source in the system and thework object.
 21. The method of claim 20 wherein the etchant comprisesNF₃ gas.
 22. The method of claim 18 wherein the surface at the lowertemperature end of the temperature differential is a surface on acomponent of the temperature measurement system.
 23. A method ofcleaning an RTP process chamber, comprising: heating a selectedabsorbent surface in the process chamber with energy from a radiantenergy source, the radiant energy passing through a transmissive surfacebetween the radiant energy source and the selected surface; allowing aselected transmissive surface in the chamber to heat by energytransferred from the selected absorbent surface, after the absorbentsurface is heated by the radiant energy source; and contacting theheated transmissive surface with an etchant while there is apredetermined temperature differential between the selected transmissivesurface and another selected surface in the chamber; and allowingsufficient time for the etchant to react preferentially at thetransmissive surface to a predetermined degree relative to the otherselected surface.
 24. The method of claim 23 wherein the etchantcomprises NF₃ gas.
 25. The method of claim 23 wherein the other selectedsurface is on a component of a temperature measurement system.
 26. Themethod of claim 23 wherein deposits comprising polysilicon are on thetransmissive surface.
 27. The method of claim 23 wherein the otherselected surface is subject to cooling by a cooling means so that it isat the lower end of the temperature differential.
 28. A method of insitu cleaning of a process chamber, comprising: running productionwafers through a process chamber; stopping production runs for cleaningthe chamber after a set number of wafers have been processed in thechamber; heating an absorbent surface in the process chamber with energyfrom a radiant energy source, the radiant energy passing through atransmissive surface between the radiant energy source and the absorbentsurface; allowing a selected transmissive surface in the chamber to heatby energy transferred from the absorbent surface, after the absorbentsurface is heated by the radiant energy source; contacting the selectedtransmissive surface with an etchant after a predetermined temperaturedifferential has been established between the selected transmissivesurface and another selected surface in the chamber.
 29. The method ofclaim 28 wherein the etchant comprises NF₃ gas.
 30. The method of claim28 wherein the other surface is on a component of a temperaturemeasurement system.
 31. The method of claim 28 wherein the transmissivesurface includes deposits comprising polysilicon.
 32. The method ofclaim 28 wherein the other surface is subject to cooling by a coolingmeans so that it is at the lower end of the temperature differential.33. The method of claim 29 wherein the selected absorbent surface isheated to at least about 650° C. to about 750° C. so that it transfersheat to the selected transmissive surfaces to establish the temperaturedifferential.
 34. The method of claim 33 wherein the selected absorbentsurface comprises a wafer.
 35. The method of claim 29 wherein theselected temperature differential between the surfaces is at least about200° C. to about 500° C., with the temperature of the transmissivesurface with the polysilicon at the upper end of the differential beingat least about 650° C. to 750° C.
 36. The method of claim 29 wherein theselected temperature differential between the surfaces is at least about200° C., with the temperature of the surface at the upper end of thedifferential being at least about 650° C.
 37. The method of claim 29further comprising evacuating NF₃ and etching residues from the processchamber, and resuming production runs of wafers.
 38. The method of claim28 wherein the process chamber is being used to form HSG capacitors onthe production wafers.
 39. A method of cleaning a semiconductor processchamber using a gas etchant comprising: heating an absorbent surface inthe process chamber by energy from a radiant energy source, theabsorbent surface being heated from about 400° C. to about 1500° C.;allowing a first selected surface in the chamber to heat by energytransferred from the heated absorbent surface; and cooling a secondselected surface in the chamber so that there is a temperaturedifferential between the cooled selected surface and the heated selectedsurface such that an etchant capable of reacting at both selectedsurfaces in the chamber reacts preferentially with the heated selectedsurface.
 40. The method of claim 39 wherein the etchant comprises NF₃gas.
 41. The method of claim 40 wherein the temperature differential isat least about 200° C.
 42. The method of claim 14 wherein at thepredetermined temperature differential the polysilicon/silicon dioxideetching selectivity ratio is in the range of about 4/1 to about 7/1. 43.The method of claim 42 wherein the etchant is NF₃ gas.
 44. The method ofclaim 26 wherein at the predetermined temperature differential thepolysilicon/silicon dioxide etching selectivity ratio is in the range ofabout 4/1 to about 7/1.
 45. The method of claim 23 wherein the selectedabsorbent surface is heated to at least about 650° C. to about 750° C.so that it transfers heat to the selected transmissive surfaces toestablish the temperature differential.
 46. The method of claim 45wherein the selected absorbent surface comprises a wafer.
 47. The methodof claim 26 wherein the selected temperature differential between thesurfaces is at least about 200° C. to about 500° C., with thetemperature of the transmissive surface with the polysilicon at theupper end of the differential being at least about 650° C. to 750° C.48. The method of claim 23 wherein the selected temperature differentialbetween the surfaces is at least about 200° C., with the temperature ofthe surface at the upper end of the differential being at least about650° C.